Truck Repair: F-250SD, auto hubs, hub seal

Question
I HAVE A 2000 F-250SD 4X4. WHEN I PUT IT IN 4X4 IT WILL STAY IN FOR A WHILE AND THEN THE FRONT WHEELS WILL QUIT PULLING BUT THE LIGHT ON THE DASH SAYS IT IS STILL IN 4X4. ALSO WHEN YOU TAKE IT OUT OF 4X4 THE AUTO HUBS WILL NOT UNLOCK.PLEASE HELP!!

                   THANK YOU  

Answer
Hi John,
Several variables there John.
Some of them have a vacuum actuator to lock the hubs in, which is automatic, but if there is a leak in the hub seal, or the vacuum supply is lost, it will not engage.
The light is probably operated by the transfer case, which could be engaged, even if the hubs are not.
And if either front hub is not engaging, for whatever reason, it won't pull, even though the front driveshaft is turning.
If you can get the truck in a position where the rear wheels are spinning, like a steep pile of gravel or snow, look at the front axle shaft "U"joints in the steering knuckles, while someone else is driving.
If the front driveshaft is turning, and ONE of the front drive axles is turning, that is the hub you need to check.
The axle that is not turning is probably locked in ok, but can't pull due to the differential in the front axle free wheeling the other axle shaft.

As for the not unlocking, probably the same reason.
Pull the hub cover, and see if there is muddy grease in there from water getting in.

Van